Provide way to get program name when not using glibc

If we use a compiler which is mostly GNU compatible, such as clang, and
we are on a platform which doesn't use glibc, such as MacOS X, then we
need to provide an alternative implementation of the intrinsic variable
program_invocation_name which is used in the error reporting.
Merge pull request #179 from rolk/179_exename

Enable building without using glibc (on some platforms)